High-precision optical position-measuring systems often include a plurality of high-resolution scanning units that are coupled optically to a plurality of light sources and detectors. Fiber optics are often provided for the optical coupling. The optical coupling of the various components, e.g., with the aid of fiber optics, is an important part of such a high-precision position-measuring system, since heat input may thereby be kept very low. Such high-precision position-measuring systems usually use plate-shaped, two-dimensional measuring standards in order to be able to generate position values with regard to a machine component movable in a two-dimensional XY-movement range. As a rule, the measuring standards used in this context must be calibrated very precisely. Such a calibration may be carried out by additional redundant sensors which sense calibration standards introduced into the respective machine. In addition, there is also the possibility of performing what is referred to as a self-calibration of the plate-shaped measuring standards by redundant scanning units of the position-measuring system. In this case, it is important that the expenditure for such additional scanning units be kept as low as possible. Up to now, however, this was not readily possible, especially given the necessary optical coupling by fiber optics for the additional scanning units.
German Published Patent Application No. 35 26 049 describes an arrangement or a position-measuring system in which the light from a light source is fed via a fiber optic to a time-division multiplexer in the form of a chopper wheel, which generates light pulses following one another in time at its N outputs. Consequently, a fiber optic is provided for the optical coupling, as well. The light pulses are subsequently fed in spatially separate manner to N sensors. The light pulses modulated by the sensors are united in a demultiplexer into one common detection fiber and are received by a single detector. The method described in this document substantially describes a time-division multiplexing method. The requisite time-division multiplexer, which must take the form of an active component, must be viewed as a crucial disadvantage of such a time-division multiplexing method. The time-division multiplexer may be mounted away from the sensors, the result of which, however, is that for N sensors, N optical detection channels or leads from the time-division multiplexer to the sensors are needed correspondingly, and the expenditure becomes correspondingly high. Alternatively, the time-division multiplexer could also be disposed adjacent spatially to the sensors. However, the power dissipation of the active time-division multiplexer will then prevent a high-precision position determination by the position-measuring system.
U.S. Pat. No. 5,408,091 describes a position-measuring system having optical sensors, that requires three pulsed light sources having different wavelengths, whose light pulses are coupled into one common fiber optic. In the immediate vicinity of N sensors, the light pulses are separated according to their wavelength by a fiber spectrometer. The fiber optic that transmits the light pulses at one of the three wavelengths is split in a fiber coupler into N fiber optics and directed via suitably different delay routes to the N sensors. They therefore receive light pulses following one another in time. The light pulses modulated by the sensors are likewise led off via fiber optics and the fiber optics are combined in a fiber coupler to form a single fiber optic. The downstream detector therefore receives light pulses following one after another in time, which are assigned to the individual sensors. Consequently, this method again represents a time-division multiplexing method, the time shift here being accomplished by suitably different delay routes. Because of the speed of light, such a time shift requires very long lengths of the fiber optics in order to ensure a time interval between the light pulses which is detectable electronically. The expenditure and the space requirements for such a position-measuring system are correspondingly high.
A position-measuring system similar to this is also described in U.S. Pat. No. 5,498,867. In elaboration of the principle described in U.S. Pat. No. 5,408,091, there, the light pulses of a spectrally wide-band light source are coupled into one fiber optic and changed into M spectrally narrow-band light pulses in a fiber spectrometer. These spectrally narrow-band light pulses are fed via separate fiber optics to M groups of sensors. Each of the M groups of sensors contains one fiber coupler which in each case splits the fiber optic into N fiber optic and directs them via different delay routes to the N sensors of each group. Therefore, M×N sensors are supplied by one light source and one fiber optic, respectively. The light pulses modulated by the M×N sensors are united via a fiber coupler in one common fiber optic and ultimately fed to a spectrometer which conducts the M spectrally narrow-band light pulses to M detectors. Each of the M detectors detects the associated group of sensors in a time-division multiplexing method. Therefore, the method described in U.S. Pat. No. 5,498,867 may be referred to as a mixed time-division and wavelength multiplexing method. Due to the different wavelengths, the sensors must be adapted for these wavelengths. However, in a position-measuring system having a single plate-shaped measuring standard scanned by all scanning units, such an adaptation is not possible, since the grating structure (especially the phase height) of the measuring standard can only be optimized for a single wavelength.